26 A HISTORY OF BIRDS 



could be drawn therefrom, did space permit, of the way in 

 which muscles, originally single, become split up to form two 

 or more distinct bodies, which in turn undergo further modifi- 

 cation and often degeneration. Both the wing and the thigh 

 furnish examples of this kind. Two instances thereof must 

 suffice. 



The first of these shall be taken from the wing, and shall be 

 confined to a single group of birds, the Passeres. The muscle 

 selected is that known as the deltoidcns, which, in its primitive 

 " archicentric " condition, arises in part from the acromion of 

 the scapula and the inner, free face of the furcula, and in part 

 from the os huniero scapularc and the crista lateralis of the 

 humerus. Running down the arm in the form of a thick 

 " belly," it is inserted into the ectepicondylar process of the 

 humerus, e.g., the Birds of Paradise. The evolution of this 

 into two distinct muscles can be easily traced. As specialisa- 

 tion proceeds the belly splits into two portions, the origin and 

 insertions remaining the same, as in the Common Starling. 

 This is the beginning of the transformation into distinct 

 longus and brevis portions. In the Corvida^ the longus portion 

 becomes degenerate, fusing with the brevis portion which is 

 continued downwards to terminate in a tendon at the usual 

 point of insertion. In the primitive "Broad-bills" iyEurylce- 

 niidiE), on the other hand, it is the brevis portion which is 

 degenerate, and which terminates on the upper third of the 

 humerus shaft, leaving the longus portion to effect the usual 

 union with the distal end of the humerus. What factors have 

 brought about this splitting of the muscle, and what have de- 

 termined the survival of the brevis portion in one and of the 

 longus portion in another, no man can say. Nor does it seem 

 that these changes can have any bearing on the problems of 

 existence. 



A similar illustration is furnished by the " Ambiens" muscle 

 of the thigh — a reptilian heritage. In its fully developed con- 

 dition this muscle arises from a projecting spine on the pelvis, 

 beneath and in front of the acetabulum — the pectineal process 

 — and descending the inner surface of the thigh crosses the knee- 

 cap in the form of a tendon, and turning downwards joins the 

 perforated flexor muscle for the second, third and fourth digit 

 {Flexor perforans iudicis, niedii and annularis), splitting up for 



